Now, 40 years later, Earth Day is every day, as the saying goes. The thing is, it's also everyday—environmentalism has become a routine, if not universally embraced, part of U.S. culture, with green-ness as much a marketing tactic as a moral pursuit.

"I think the novelty [of Earth Day] has worn off," said Steve Cohen, the executive director of the Earth Institute at Columbia University.

"We're not burying cars or doing those kinds of dramatic gestures anymore," Cohen said. "Originally, it was seen as an expression of a radical political belief. It's hard to argue that it's radical anymore."

Earth Day has been tamed, Cohen said, but that's not to say it isn't still relevant. Earth Day serves as a useful yearly reminder for people to consider their impact on the environment, he said.

"In our very fast paced society, [Earth Day] helps to get people to step back and think about things a little bit differently," he said.

But Earth Day is about more than just thinking, said Timmons Roberts, director of the Center for Environmental Studies at Brown University.

Earth Day provides a rallying point for people to address the unique environmental problems of their time, Roberts said.

For example, "each generation of college students reinvents [Earth Day] for the issues of their day," he said. At Brown, students these days focus largely on global wamring and reducing energy consumption, whereas in the past they might have been more likely to speak out on deforestation, chemicals that deplete the ozone layer, river pollution, or smog, he said.

Earth Day "really does bring people together and creates a community of people who care and who want to do something, even if it's just picking up trash in their neighborhood," he said. "I think we do need it."

Earth Day also continues to be a good introduction to environmentalism for novices, said Kathleen Rogers, president of Earth Day Network, a nonprofit group that aims to broaden the environmental movement and mobilize environmentalists worldwide.

Once upon a time, environmental advocacy was viewed by many as an elitist pursuit and the domain of high society, Rogers said.

"Before 1970 the movement was largely focused on land and species conservation," she said. That year "marks the line really clearly between the small group of people who were protecting the environment for hunting, shooting, and beauty purposes, and the rest of the world."

A shift was exactly the point of the first Earth Day, 40 years ago, Rogers said.

U.S. Senator Gaylord Nelson and activist Denis Hayes, who organized the first Earth Day, deliberately focused on the effects of the environment on human health—a topic relevant to everyone.

Earth Day "allowed the world to find a home in environmentalism," Rogers said. "It was an entry point for hundreds of millions of people ... Everybody had a stake all of a sudden."

That change in thinking was a "profound cultural shift," added Hayes, now president and CEO of the Seattle-based Bullitt Foundation, which promotes sustainable development in the U.S. Pacific Northwest.

"Today, every American believes he or she has a fundamental right to a safe and healthy environment, and that nobody has a right to put something in that environment that adversely affects them or their children or their neighbors," Hayes said.

Earth Day going mainstream does have some downsides, however, said Brown University's Roberts.

"The first Earth Day was effective because so many people went out in the streets," Roberts said.

Forty years later, "people are asked to do much simpler things, like recycle or turn their thermostat to a certain level," he said. "They're not being asked to get out there and shake up the government and force a recognition of how things are produced and how much we consume."

Roberts draws a direct connection between today's tamer Earth Day activities and what he sees as the United States' ineffectiveness in the face of environmental problems such as global warming.

"There isn't the same level of civil pressure on the government to do anything about it. ... I think as the movement became professionalized, people were satisfied to recycle and send in their checks to somebody else to let them do the agitation," Roberts said.